This invention relates to a three-dimensional image display apparatus and direction-selective reflection screens therefor.
Three-dimensional images can be displayed by projecting a multiplicity of elemental images having parallax information onto a direction-selective reflection screen. One example of a photogram consisting of a multiplicity of independent and non-overlapping elemental images involves a multiplicity of elemental pictures, recorded in respective film frames of the same scene photographed from different angles with a stereoscopic camera, or a plurality of standard cameras coupled to create parallax. Each of the elemental pictures corresponds to an image of a large portion of the scene being photographed as viewed by one of a pair of human eyes positioned at a parallactic distance from the other. In other words, the multiplicity of elemental pictures has parallax information. In most cases, the parallax direction is horizontal. One way of producing a three-dimensional image by projection of the aforementioned elemental pictures is to set up a number of projectors corresponding to the number of the elemental pictures and arranged in the same way as the camera objective lenses were arranged when the pictures were taken, and direct the projector at the same direction-selective reflection screen so it can be observed at a predetermined position relative to the projectors. Of course, it is possible to replace the number of projectors with a stereoscopic projector having a number of projection objectives.
In this specification, the direction between photograhic objectives during photography of the elemental pictures or the direction between projecting objectives during projection of the above-mentioned elemental pictures is known as the parallax direction. Specifically, the direction connecting the eyes of a photographer or the eyes of an observer is the parallax direction. In most cases, when the line connecting the photographer's eyes or the observer's eyes is horinzontal, the parallax direction (or parallactic direction) is usually horizontal.
Various types of direction-selective reflection screens are known. For example, that disclosed in U.S. Pat. No. 1,883,291. This screen is constructed of a network of reflector elements, each of which consists of two mirrors arranged to intersect at right angles to each other in a line perpendicular to the parallax direction of the image to be projected thereon. A disadvantage of such a screen in projecting the elemental images is that as the axis of the light beam projected departs farther and farther away from the axis normal to the planes parallel to the intersection of the two mirrors, the incident beam is reflected further upward or downward. Hence, a large proportion of the reflected beam will not reach the eyes of an observer located at a position corresponding to that of the projector. As a result, the brightness of the three-dimensional image particularly at the upper and lower portions thereof is unacceptably reduced. To avoid this phenomenon, it is necessary that the entire surface of the screen be curved so as to be cylindrical with respect to the line parallel to the parallax direction. Such a deformation of the screen, however, will result in an appreciable distortion of the displayed image.
There is another way to construct the screen having improved reflection selectivity characteristics without imparting any curvature to the entire surface of the screen. This is accomplished with a network of so-called cubic corner type reflectors, each of which consists of three flat reflecting surfaces arranged to intersect at a common point with the reflecting surfaces at right angles to each other. A light beam incident upon such a screen is reflected back along the incident beam accurately upon the source of the light beam. Therefore, a three-dimensional image, though displayed on the screen by projecting the elemental images in the photogram having parallax information, cannot be observed unless the observer is located exactly at the projector. With a half-mirror positioned in the path of the projection light beam between the projector and the screen, the three-dimensional image is made observable to the observer only when he is located at the position symmetrical to the position of the projector with respect to the half-mirror plane; that is, at the position corresponding optically to the position of the projector. It follows that the three-dimensional image display system employing the cubic corner type reflector screen is characterized by an extremely narrow range of optimum observer positions. In order for the display system to permit some freedom of motion of the head in the vertical direction from the projector in observing the display, it is necessary to accommodate a number of additional projectors superimposed on the existing projector in a plane perpendicular to the parallax direction. This additional projector arrangement defeats the purpose of minimizing the complexity of the three-dimensional image display apparatus. Further, the utilization of the half-mirror arranged between the projector and the screen causes a considerable loss of image-forming light in its reflection therefrom as well as in its transmission therethrough to the observer.
Cubic corner reflector screens are disclosed in U.S. Pat. Nos. 1,671,086 and 1,743,835. These screens are used for purposes of road signs, etc. and have one or more surfaces on each reflector deformed to spread light in all directions. If such screens were used with adjacent projectors projecting complementary parallax images, the light on the screen would be spread in all directions simultaneously. Thus, the light from one projector projecting one parallax image becomes confused with the light from another projector projecting another parallax image over the entire screen. Thus, a three-dimensional image would not be perceived.